This invention relates to the fabrication of a micromechanical device for spatially and temporally modulating an incident beam of light by diffraction. More particularly, this invention discloses an improved method for manufacturing an electromechanical device with a conformal grating structure to produce high contrast and increased device life.
Electromechanical spatial light modulators with a variety of designs have been used in applications such as display, optical processing, printing, optical data storage, and spectroscopy. These modulators produce spatial variations in the phase and/or amplitude of an incident light beam using arrays of individually addressable devices.
One class of electromechanical spatial light modulators has devices with a periodic sequence of reflective elements that form electromechanical phase gratings. In such devices, the incident light beam is selectively reflected or diffracted into a number of discrete orders. Depending on the application, one or more of these diffracted orders may be collected and used by the optical system. Electromechanical phase gratings can be formed in metallized elastomer gels; see U.S. Pat. No. 4,626,920, issued Dec. 2, 1986 to Glenn, and titled, xe2x80x9cSolid State Light Modulator Structurexe2x80x9d and U.S. Pat. No. 4,857,978, issued Aug. 15, 1989 to Goldburt et al., and titled xe2x80x9cSolid State Light Modulator Incorporating Metallized Gel And Method Of Metallization.xe2x80x9d The electrodes below the elastomer are patterned so that the application of a voltage deforms the elastomer producing a nearly sinusoidal phase grating. These types of devices have been successfully used in color projection displays.
An electromechanical phase grating with a much faster response time can be made of suspended micromechanical ribbon elements, as described in U.S. Pat. No. 5,311,360, issued May 10, 1994, to Bloom et al., and titled, xe2x80x9cMethod And Apparatus For Modulating A Light Beam.xe2x80x9d This device, also known as a grating light valve (GLV), can be fabricated with CMOS-like processes on silicon. Improvements in the device were later described by Bloom et al. that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbons and the substrate, and 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast. See U.S. Pat. No. 5,459,610, issued Oct. 17, 1995, to Bloom et al., and titled, xe2x80x9cDeformable Grating Apparatus For Modulating A Light Beam And Including Means For Obviating Stiction Between Creating Elements And Underlying Substrate.xe2x80x9d Bloom et al. also presented a method for fabricating the device; see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, to Bloom et al., and titled xe2x80x9cMethod Of Making A Deformable Grating Apparatus For Modulating A Light Beam And Including Means For Obviating Stiction Between Grating Elements And Underlying Substrate.xe2x80x9d Additional improvements in the design and fabrication of the GLV were described in U.S. Pat. No. 5,841,579, issued Nov. 24, 1998 to Bloom et al., and titled, xe2x80x9cFlat Diffraction Grating Light Valvexe2x80x9d and in U.S. Pat. No. 5,661,592, issued Aug. 26, 1997 to Bornstein et al., and titled, xe2x80x9cMethod Of Making And An Apparatus For a Flat Diffraction Grating Light Valve.xe2x80x9d
Previously mentioned linear GLV arrays have a diffraction direction that is not perpendicular to the array direction, and thus increases the complexity of the optical system required for separating the diffracted orders. Furthermore, the active region of the array is relatively narrow, hence requiring good alignment of line illumination over the entire length of the array, typically to within 10-30 microns over a few centimeters of length. The line illumination then also needs to be very straight over the entire linear array.
There is a need, therefore, for a linear array of grating devices that has a large active region with the diffraction direction perpendicular to the array direction. Furthermore, the device must be able to diffract light efficiently at high speed into discrete orders and the device fabrication must be compatible with CMOS-like processes.
The need is met according to the present invention by providing a method of manufacturing a mechanical grating device, including the steps of: forming a spacer layer on a substrate; removing portions of the spacer layer to define an active region; forming a sacrificial layer in the active region; forming conductive reflective ribbon elements over the active region; annealing the mechanical grating device at an annealing temperature greater than 150xc2x0 C.; and removing the sacrificial layer from the active region to release the conductive reflective ribbon elements.